Photosynthesizers
began their evolution in the
sea. It is thought that chloroplasts,
the bodies in plant cells that
do photosynthesis, were once
small independent
bacteria.

Chloroplasts
moved into larger cells, and
those larger cells provided
a nurturing environment for
the chloroplasts. In return,
the chloroplasts used water,
carbon dioxide, and sun light
to make sugars. By capturing
the energy of the sun, the chloroplasts
made energy available for plant
and seaweed growth, and, through
these autotrophs (organisms
that create their own food),
for all the animals that live
on the planet.

Life
in water is much easier
for seaweeds than life
on land is for
plants. In water,
seaweeds are supported, kept
moist, and carried to
new locations.
Photosynthesizers lived in the
oceans for many millions
of years before venturing
onto the land.

Water
plants can be delicate
and ribbon-like or rather
rubbery. Water plants
are supported by the
water that they live
in: they are buoyant.
They do not need structures
to hold them up, because
the water supports them.
They do have to deal
with currents in the
water, so some of them
are rather tough (like
kelp stalks, for instance).

In
this picture of an ocean
plant you can see the
tough, rubbery stalk.
Above it there are small
oval structures like
little balloons. These
oval structures are
filled with gas and
tend to float in the
water: they help to
hold the stalk up. Above
the ovals are the long,
thin ribbons of plant
tissue that float towards
the surface, towards
the light. The cells
in these ribbons of
plant tissue contain
chloroplasts that make
nourishing sugars from
sunlight, water, and
carbon dioxide.

Seaweeds
do not need roots,
because they have water
around them all the
time. The individual
cells just help themselves
to what they need. However,
some water plants have
evolved holdfasts
to allow them to
cling to rocks. Holdfasts
do not transport anything,
they just hold the base
of the plant in one
place.

Vascular
Structures

Plants
on land have vascular
structures, internal
tubes that allow water
to go to every part of
the plant, and that allow
the food energy to go
down to the roots as needed.
Seaweeds may be
ribbon-like and only two
big cells thick: in a
double layer of cells,
every cell has access
to the water around it,
and vascular structures
are not needed.

Reproductive Strategies

Seaweeds
do not have seeds as
land plants do. They
can reproduce
themselves by simply
breaking apart, or
they can release eggs
and sperms to make
spores, tiny
one-celled capsules
that can float away
and grow into seaweeds
by themselves.
Seaweeds do not have
flowers.